1. Field of the Invention
The present invention relates to an image display device for displaying a color image on a color display unit and a method for adjusting correction data in a look-up table (which stores correction data for correcting color balance of a color image and will be hereafter referred to as LUT).
2. Description of the Related Art
Image display devices for displaying color images on a color display unit in the prior art include a so-called liquid crystal display (hereafter referred to as LCD) television, which receives a television signal broadcast from a television broadcast station and displays a color image based on the television signal on an LCD panel as a color display unit. The LCD television comprises an LUT which stores correction data for correcting color balance (hue) of an image to be displayed on the LCD panel (so as to achieve predetermined color balance). It is designed that an image based on a television signal is corrected for color balance based on the correction data in the LUT (so as to achieve predetermined color balance), and then is displayed on the LCD panel. More specifically, such an LCD television is designed to use the correction data in the LUT to correct an input signal value representing a brightness level of each color of red, green and blue in image data of the image based on the television signal (i.e. image data generated from the television signal), so as to display a color image based on the corrected input signal value on the LCD panel.
Note that correction data in an LUT is calculated and generated using one LCD television called average product and based on color balance of an image displayed on this LCD television. However, there are individual differences among individual LCD televisions. This causes differences in color balance (hue) among resultant images displayed on individual LCD televisions even if the color balance of an image based on the same image data is corrected in the same way based on the same correction data in the LUT in the individual LCD televisions. This means that a color balance of a displayed color image after corrected based on correction data in an LUT may, in some cases, not be a predetermined color balance due to individual differences among individual LCD televisions.
Thus, an adjustment called white balance adjustment is made in such LCD televisions before shipment in order to eliminate differences in color balance or hue of displayed images due to individual differences among individual LCD televisions, or more specifically to bring a color balance of a displayed color image to a predetermined color balance when the color balance of the displayed image after corrected based on correction data in an LUT is not the predetermined color balance due to the individual differences among the individual LCD televisions. A conventional LCD television is designed such that a gain and an offset are provided to an input signal value representing a brightness level of each color of red, green and blue in given image data (image data generated from a television signal), and each input signal value provided with the gain and the offset is corrected based on correction data in an LUT so as to display a color image on an LCD panel based on each corrected input signal value, and that the white balance adjustment is made (i.e. color balance of the displayed image is brought to a predetermined color balance) by adjusting the gain value and the offset value provided to the input signal value.
The conventional LCD television comprises a gain adjustment unit for providing gain to an input signal value and an offset adjustment unit for providing offset to the input signal value. The white balance is adjusted by adjusting a gain value of the gain adjustment unit (more specifically, a gain value provided to the input signal value) and adjusting an offset value of the offset adjustment unit (more specifically, an offset value provided to the input signal value) without replacing the LUT by another or updating the correction data of the LUT. This white balance adjustment is made by displaying a predetermined white balance adjustment image (white color or neutral color image with a predetermined brightness level) on the LCD panel, and by adjusting the gain value of the gain adjustment unit and the offset value of the offset adjustment unit so as to bring the color balance of the displayed white balance adjustment image to a predetermined color balance.
More specifically, image data of a white balance adjustment image is provided to the LCD television from a white balance adjustment image supply device as an external device, so as to provide a gain and an offset (gain of 1 and offset of 0 in an initial state) to an adjustment input value (input value used for adjustment) which is an input signal value representing a brightness level of each color of red, green and blue in the image data of the white balance adjustment image provided to the LCD television. The adjustment input value provided with the gain and the offset is corrected based on the correction data in the LUT, and the white balance adjustment image based on the thus corrected adjustment input value is displayed on the LCD panel. The color balance of the white balance adjustment image displayed on the LCD panel is measured by a measuring device as an external device. The gain value of the gain adjustment unit and the offset value of the offset adjustment unit are adjusted so as to allow the measured color balance value to be in a predetermined range, i.e. bring the color balance of the white balance adjustment image displayed on the LCD panel to a predetermined color balance.
Even more specifically, as the white balance adjustment image, two white balance adjustment images with different brightness levels (the white balance adjustment image with a lower brightness level being hereafter referred to as Low side white balance adjustment image, and the white balance adjustment image with a higher brightness level being hereafter referred to as High side white balance adjustment image) are displayed on the LCD panel. The gain value of the gain adjustment unit is adjusted to bring the color balance of the High side white balance adjustment image to a predetermined color balance, while the offset value of the offset adjustment unit is also adjusted to bring the color balance of the Low side white balance adjustment image to a predetermined color balance. This is done by repeating a process of displaying the High side white balance adjustment image on the LCD panel and adjusting the gain value of the gain adjustment unit, and a process of displaying the Low side white balance adjustment image on the LCD panel and adjusting the offset value of the offset adjustment unit, whereby the gain value of the gain adjustment unit and the offset value of the offset adjustment unit are adjusted so as to bring the color balance of each of the High side white balance adjustment image and the Low side white balance adjustment image to a predetermined color balance. The white balance adjustment is made in this way.
In order to display a color image based on a television signal after the white balance adjustment is made, the thus adjusted gain value and the offset value are used to provide a gain and an offset to the input signal value representing a brightness level of each color of red, green and blue in the image data of the color image to be displayed, and each input signal value provided with the gain and the offset is corrected based on the correction data in the LUT so as to display a color image on the LCD panel based on each corrected input signal value. Here, the gain value and the offset value after adjusted by the white balance adjustment are equally applied to the entire range of the input signal value, so that a gain and an offset based on the thus adjusted gain value and offset value are equally provided to the input signal value representing a brightness level of each color of red, green and blue in the image data of the image to be displayed.
However, the conventional white balance adjustment described above has the following problems. (1) First, as described above, an offset is equally provided to an input signal value representing a brightness level of each color of red, green and blue in image data of an image to be displayed. Thus, when the input signal value is low (particularly when it is lower than the Low side adjustment input value), the color accuracy of the displayed image is lowered (the color balance of the displayed image is caused to shift from the predetermined color balance). For example, when the offset value provided to the input signal value representing the brightness level of red is a positive value, the displayed image is caused to take on reddish color, more than expected, in dark areas (areas of the displayed image with low brightness) including black.
(2) Second, the gain adjustment by displaying the High side white balance adjustment image and the offset adjustment by displaying the Low side white balance adjustment image influence each other. This increases time for the adjustment because it is required to repeat many times the offset adjustment process to bring the color balance of the Low side white balance adjustment image to a predetermined color balance as well as the gain adjustment process to bring the color balance of the High side white balance adjustment image to a predetermined color balance until the color balance of both Low side and High side white balance adjustment images is brought to a standard value (predetermined color balance) (more specifically, until it becomes possible to determine the gain value and the offset value which bring the color balance of both Low side and High side white balance adjustment images to the predetermined color balance).
Thus, according to the conventional white balance adjustment, displayed images have low color accuracy after the white balance adjustment. In other words, the white balance adjustment has low accuracy. Further, it requires a long time for the white balance adjustment. Although the problems of white balance adjustment have been described above using an LCD panel as an example, similar problems can be found in other image display devices for displaying color images on a color display unit such as plasma display panels, CRT (Cathode Ray Tube) display panels and organic EL (Electroluminescence) display panels. The following describes some other technologies of white balance and other adjustments in the prior art.
Japanese Laid-open Patent Publication Hei 5-236498 discloses an automatic white balance adjusting device which sets a reference gain data and a reference DC offset data as an initial value to be used as a reference for white balance adjustment. It detects a gain error between the reference gain data and a bright screen data which is an electrical signal converted from a bright screen determining a gain, and also detects an offset error between the reference DC offset data and a dark screen data which is an electrical signal converted from a dark screen determining a DC offset. The gain and the offset are controlled to null the gain error and the offset error so as to make white balance adjustment.
Japanese Laid-open Patent Publication 2001-238227 discloses a method of tone correction. It measures contrast, and sets a dynamic range of contrast which can be used for display, and further makes gain adjustment and offset adjustment adapted to the dynamic range. Thereafter, it recalculates and writes data in an LUT to achieve full dynamic range, thereby generating tone correction data. Further, Japanese Laid-open Patent Publication Hei 7-64522 discloses an automatic adjusting system for a multi-display device which has multiple display units combined to form a large screen display, and which makes white balance adjustment by allowing a camera to detect amounts of red, green and blue light from the multiple display units. However, such technologies disclosed in the prior art do not solve the above problems.